In such a high pressure discharge lamp, the so-called foil seal structure, in which a base portion of an electrode axis is joined to a metallic foil buried in a sealing portion, is adopted as a sealing structure. In general, the electrode axis of the electrode is made of tungsten while an arc tube is made of silica glass, thus the sealing portion of the arc tube often breaks or is damages occurs due to difference in the thermal expansion coefficient in the sealing portion. This becomes a more serious problem, especially, in a high pressure discharge lamp that contains a large amount of mercury, i.e. 0.15 mg/mm3 or more, enclosed in a light emitting portion since the mercury steam pressure increases, i.e. 100 or more atmospheric pressure, at time of lighting.
In order to solve such a problem, Japanese Patent Application Publication No. 2008-529252 teaches technology in which grooves are formed on an electrode axis (rode core) extending in an axial direction thereof. FIG. 3A is a schematic diagram of the structure of a lamp according to the above-mentioned example of the prior art, and FIG. 3B is an enlarged view of an electrode. As shown in FIGS. 3A and 3B, two or more grooves 5, which extend in the direction of an axis thereof, are formed on an outer surface area of an electrode axis 21 of each electrode 2 provided in a discharge lamp 1. In addition, each electrode axis 21 is connected to a metallic foil 4 in the sealing portion 3. In the above-mentioned conventional technology, the surface roughness in a circumference direction is made larger than that of a longitudinal direction thereof by forming grooves on the electrode axis, thereby preventing breakage of the sealing portion due to the difference in the thermal expansion coefficient of the materials.